Zayd ibn Harithah was a boy from a tribe of Yemen. When he was about seven or eight, his mother Su'da bint Tha'laba took him to visit her own tribe. While they were there, a fight broke out between his father's tribe and his mother's tribe. Some distant relatives on his mother's side were so angry that they kidnapped Zayd and sold him into slavery. They did this to take revenge on his father's tribe, because before Islam a son belonged to his father's lineage.
Zayd was sold at the great fair of Ukkadh, the largest market of the Arabs, held after the hajj season. Khadija bint Khuwaylid RA had given her nephew Hakim ibn Hizam money to find her a young slave. Hakim bought Zayd for 400 dirhams, and Zayd became Khadija's servant. This was years before her marriage to Muhammad ﷺ.
This is how Zayd came into Khadija RA's household. When she later married Muhammad ﷺ, she gave Zayd to him, and the two formed one of the closest father-son bonds of the Prophet ﷺ's life. The kidnapping also set off his father Harithah's long search for him. That search led to the offer to ransom him and, in the end, to Zayd's adoption.
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